In situ polymerization of a novel surfactant on a graphene surface for the stable dispersion of graphene in water

Abstract

A pristine graphene surface is functionalized with a novel polymerizable surfactant by in situ polymerization with a crosslinker. The in situ polymerization immobilizes the surfactant molecules on the graphene surfaces, which allows the stable dispersion and redispersion of graphene in water. The resultant polymerized-surfactant/graphene is then shown to be useful as a conductive graphene ink.

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Carbon nanomaterials have been explored as an emerging class of functional nanomaterials since the end of the last century. In the past decade, graphene has attracted much attention from materials science and material engineering because of its excellent electronic, optical, thermal and mechanical characteristics.^1–9 Owing to the strong interaction between graphene sheets and to its high hydrophobicity, a challenge remains as to the handling of graphene in large quantities for industrial applications. To date, there have been many efforts to produce and process graphene for application.^9–11 One of the scalable and promising approaches is colloidally dispersed graphene in solvents.^10–15 In fact, many reports have described that surfactants are effective for the colloidal dispersion of graphene without producing defects.^9,16–24 Since the dispersion by surfactants is based on the reversible physical adsorption of surfactant molecules on graphene surfaces, it requires an excess amount of surfactants, which might affect the properties of graphene, especially for their applications in electronic devices and sensors. Another obstacle is that the interaction between a surfactant and graphene can easily be disrupted by external stimuli, such as another type of interface (e.g., solid polymer surface and oil/water interface), dilution, thermal stimuli and extraction with organic solvents.

Polymerization of surfactant molecules on a substrate surface is a rational approach to “permanently” and “noncovalently” immobilizing surfactant molecules on a surface.^25,26 This approach does not damage a substrate chemically and keeps the intrinsic characteristics of a pristine substrate, which are advantageous to many nanomaterials. Kang and Taton reported the successful immobilization of polymer surfactants on the surfaces of carbon nanotubes (CNTs) through crosslinking and the redispersion of the crosslinked-polymer/CNTs.²⁷ Choi et al. also succeeded in the in situ polymerization of a surfactant-like monomer on the surfaces of CNTs, which was also redispersible in water.²⁸ Fernandez et al. functionalized the surface of a carbon nanotube using a novel polymerizable glycolipid and demonstrated protein immobilization on the polymerized lipid shell encapsulating CNTs.²⁹ In the present study, we synthesized novel polymerizable surfactants and succeeded in the in situ polymerization of the surfactant molecules on graphene surfaces (Fig. 1a), which contributed to the stable graphene dispersion in water. To our knowledge, this is the first report of in situ polymerization of surfactant molecules on pristine graphene surfaces.

Fig. 1 (a) Schematic illustration of the in situ polymerization of polymerizable surfactant molecules on a graphene surface. (b) Chemical structures of precursors (non-polymerizable surfactants) and polymerizable surfactants.

We designed polymerizable surfactants to possess a polymerizable group between a hydrophobic part and a hydrophilic part to be accessible for a radical initiator in water (Fig. 1b). The hydrophobic part was a long linear alkyl chain and the hydrophilic part consisted of two triethylene glycol chains to form a micellar structure in water to facilitate a high solubility in water. The polymerizable group was ethyl acrylate that could participate in free radical polymerization with an appropriate crosslinker in water.

First, we synthesized precursors of polymerizable surfactants (non-polymerizable surfactants), Cn-2EG₃ (n = 14, 16, 18; Fig. 1b). Then, polymerizable surfactants, Cn-ac-2EG₃ (n = 14, 16, 18; Fig. 1b), were synthesized from Cn-2EG₃. The synthesis procedures are given in the ESI (Fig. S1†). The dispersion of graphene in water was investigated using the non-polymerizable and polymerizable surfactants. Fig. 2 shows the photographs, absorbance at 660 nm and the concentrations of graphene dispersed with C14-2EG₃, C14-ac-2EG₃, C16-2EG₃, C16-ac-2EG₃, C18-2EG₃ and C18-ac-2EG₃. There were obvious differences in the dispersion of graphene in water among the surfactants tested. Graphene with C14-2EG₃ showed abs = 0.256 at 660 nm, which indicates that graphene was dispersed in water to some extent (18.2 μg mL⁻¹). The polymerizable surfactant C14-ac-2EG₃ yielded a transparent solution with low absorbance at 660 nm (less than 0.08), meaning that it failed to disperse graphene in water. The dispersions of graphene with C16-2EG₃ and C16-ac-2EG₃ showed considerably high absorbance at 660 nm and higher concentrations of graphene (around 30 μg mL⁻¹) were obtained compared with C14-2EG₃ and C14-ac-2EG₃. The dispersion of graphene with C18-2EG₃ and C18-ac-2EG₃ showed a higher absorbance at 660 nm than those of C16-2EG₃ and C16-ac-2EG₃. Among the 6 kinds of surfactants tested, C18-ac-2EG₃ yielded the highest concentration (52 μg mL⁻¹) of graphene dispersion in water. The longer the hydrophobic part was of the surfactant, the greater the ability to disperse graphene. These results indicate that strong hydrophobic interaction between graphene and the surfactant was required to disperse graphene in water. The following experiments were carried out using C18-ac-2EG₃ to investigate the graphene dispersion.

Fig. 2 Photographs, absorbances at 660 nm and concentrations of graphene aqueous dispersions using non-polymerizable and polymerizable surfactants before in situ polymerization. "ac" represents an acrylate group.

The in situ polymerization of C18-ac-2EG₃ on the surface of graphene was carried out in water using ammonium peroxodisulfate and N,N,N′,N′-tetramethylethylenediamine as initiators. In the polymerization process, a crosslinker (N,N′-methylenebisacrylamide (MBAA) or divinylbenzene (DVB)) was added to the reaction mixture for preparation of the cross-linked network of the polymerized-surfactant on the surface of graphene. Fig. 3 shows the photographs, absorbance at 660 nm and concentrations of graphene dispersion using C18-ac-2EG₃ before and after polymerization. The dispersion of graphene after polymerization with and without crosslinkers had a similar absorbance (around 0.7 at 660 nm) to each other, indicating that the polymerization process did not affect the concentrations of dispersed graphene.

Fig. 3 Photographs, absorbances at 660 nm and concentrations of graphene aqueous dispersions using C18-ac-2EG₃ and crosslinkers after in situ polymerization. Washing was carried out with CHCl₃.

The non-polymerized-surfactant/graphene and the polymerized-surfactant/graphene were freeze-dried and washed with CHCl₃ to remove non-immobilized surfactants from the graphene surfaces. The washed non-polymerized- and polymerized-surfactant/graphene were added in water and ultrasonicated to redisperse the graphene. Fig. 3 also shows the photographs and the absorbance of the graphene redispersed in water. The low absorbance less than 0.03 means that the non-polymerized-surfactant/graphene and the polymerized-surfactant/graphene without a crosslinker failed to redisperse graphene in water. These results suggest that the non-polymerized-surfactant and the polymerized-surfactant without a crosslinker were removed from the graphene surfaces by the washing procedure. The polymerized-surfactant/graphene with MBAA succeeded in the redispersion of graphene at 10.4 μg mL⁻¹. The redispersion test suggests that the polymerized C18-ac-2EG₃ (p-C18-ac-2EG₃) with MBAA would be immobilized on the graphene surfaces, probably owing to its crosslinked polymer network encapsulating graphene or to its multiple-point adsorption to graphene. The use of DVB as a crosslinker for the polymerization of C18-ac-2EG₃ failed to redisperse graphene after washing. This is proposed to be because of the different reactivity of DVB with the acrylate monomers. It should be noted that the washing procedure can be done using other kinds of organic solvents (e.g. toluene etc.) that can dissolve non-immobilized surfactants.

To verify the polymerization of C18-ac-2EG₃ under the present conditions, the p-C18-ac-2EG₃/graphene with MBAA was added in dichloromethane (CH₂Cl₂), followed by ultrasonication for 10 min. The CH₂Cl₂ phase was subjected to matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (Fig. S2†). The MS chart reveals the presence of p-C18-ac-2EG3 with MBAA, which supports our hypothesis of the surfactant polymerization with a crosslinker on graphene surfaces.

TEM images of the non-polymerized C18-ac-2EG₃/graphene, the p-C18-ac-2EG₃/graphene and the washed p-C18-ac-2EG3/graphene are shown in Fig. 4. Fig. 4a shows that the non-polymerized C18-ac-2EG₃/graphene was composed of a single layer or a few layers of graphene, showing the successful prevention of the aggregation of graphene sheets (more than 400 nm in diameter) by the surfactant. Fig. 4b and c shows that the polymerization of C18-ac-2EG₃ with MBAA and the washing procedure did not affect the thickness of the dispersed graphene.

Fig. 4 TEM images of (a) C18-ac-2EG₃/graphene, (b) p-C18-ac-2EG₃/graphene with MBAA, and (c) washed p-C18-ac-2EG₃/graphene with MBAA.

Mass percentages of graphene and C18-ac-2EG₃ in the washed p-C18-ac-2EG₃/graphene were evaluated by thermogravimetric analysis (Fig. S3†). Graphene and C18-ac-2EG₃ exhibited mass losses at 550–750 °C and 150–500 °C, respectively. Mass percentages of graphene and C18-ac-2EG₃ in the washed p-C18-ac-2EG₃/graphene were determined to be 2.8% and 97%, respectively. This means that the molar ratio of the carbon/surfactant was 1.8.

Fig. S4† shows the X-ray diffraction (XRD) patterns of the graphene powder, graphene non-polymerized C18-ac-2EG₃/graphene and the p-C18-ac-2EG₃/graphene. While graphite, which is stacked graphene, usually displays a sharp diffraction peak at 2θ = 26.5°,³⁰ which is characteristic of the (002) diffraction of graphite that originates from the interlayer distance between graphene sheets. The graphene dispersions in the present study showed only ambiguous and broad peaks ranging from 20–30°, which indicates that C18-ac-2EG₃ prevented the stacking of graphene before and after the in situ polymerization, although there were a few layers of graphene in the dispersion. These results are consistent with the results of the TEM observation.

Finally, we prepared the polymerized-surfactant/graphene composites on filter paper and evaluated the electrical resistance of the composites. The composites of the p-C18-ac-2EG₃/graphene and sodium dodecyl sulfate (SDS)/graphene were deposited on filter paper as graphene ink by filtrating the surfactant/graphene aqueous dispersions using filter paper (Fig. S5†). The sheet resistance of the p-C18-ac-2EG₃/graphene and SDS/graphene composites was 25 ± 5 kΩ sq⁻¹ and 32 ± 3 kΩ sq⁻¹, respectively, which were comparable with the data reported previously.^7,31–34 These results demonstrate that our present strategy using the polymerizable surfactant yields the graphene composite as conductive ink as the conventional surfactant/graphene composite.^9,31,35 It should be noted that the collection of the polymerized-surfactant/graphene composites by filtration can remove non-immobilized surfactant from the polymerized-surfactant/graphene composites.

Conclusions

In summary, we have synthesized polymerizable surfactants and succeeded in the dispersion of graphene in water using the surfactants. The in situ polymerization of the surfactant immobilized the surfactant molecules on the graphene surfaces, which allowed the stable dispersion and redispersion of graphene in water. Finally, we demonstrated that the graphene aqueous dispersion can be used as conductive graphene ink. The present strategy for the immobilization of a polymerizable surfactant on a graphene surface has the following advantages; (i) it does not impair the functional properties of pristine graphene, (ii) a graphene surface can be functionalized by surfactant molecules, (iii) the surface functionalization is irreversible and stable, and (iv) unbound and extra free surfactant molecules can be eliminated. Polymerizable surfactants will be a powerful tool for the irreversible surface functionalization of graphene without any loss of their functional properties.

Acknowledgements

We thank Prof. A. Kondo, Prof. A. Mori, Prof. S. Nishiyama and Prof. T. Nishino (Kobe Univ.) for their technical help. We thank Sekisui Chemical Co., Ltd for providing graphene and also for their valuable discussion. This work was supported partially by Special Coordination Funds for Promoting Science and Technology, Creation of Innovation Centers for Advanced Interdisciplinary Research Areas (Innovative Bioproduction Kobe), MEXT, Japan, by JSPS KAKENHI Grant Number 16H04577 and by Sekisui Chemical Co., Ltd.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The materials, experimental procedures, and additional results. See DOI: 10.1039/c6ra20315a

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